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Discussion Starter · #1 ·
On our recent trip to Florida I noticed a sound and feel that I hadn't before.

If I accelerate moderately to 47-50mph, then let off the gas, or lightly feather the brake, I heard a low boom from the front of the car. I am able to repeat this every time. I put the vehicle into engineering mode, and sure enough, the boom happens as the AFM engages going to half cylinders running. Although I dont get this sound every time the AFM engages, it ALWAYS does it under these and some other conditions. Also, I sometimes get the same boom when the system is disengaging and going to 8cyl mode under acceleration.

My question is... is this normal? I have the Superchips programmer, and can turn the feature off, but really don't want to. On the other hand, it's incredibly annoying and has me kindof concerned. I am taking it to the shop on Tuesday to have the Sunroof aligned (slightly higher on pass side), pick up some touchup paint, have a P0455 code looked at, and the AFM issue.

Any pointers on what to ask/say when I drop it off?
 

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I have a superchips tune as well and I've certainly never heard any booming sound from the AFM. I sometimes hear a low rumble before the transition happens, but that's more from engine loading before the AFM disables and the TCC releases when hitting the throttle. Would you be able to take a quick video of the sound if it's loud enough to properly pick up?
 

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Possibly:

#PIC4989: Drive Line Boom Noise And/Or Vibration - (Oct 2, 2008)

Subject: Drive Line Boom Noise and/or Vibration

Models: 2008-2009 Pontiac G8
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
There may be a either a boom noise or vibration and in some cases both a noise & vibration detected in a very few 2008 & 2009 G8's. This condition may be detected by customers at speeds of 55-65 MPH on either 08 or 09 G8's. The noise could be described as a low frequency boom or moan noise (not a whine) coming through the body. The vibration is generally felt in the seat or the floor pan. The condition could be caused by either a misaligned center support bearing assembly or the prop shaft mounting to the differential yoke.

Recommendation/Instructions:
The prop shaft center support mount is slotted left to right and can be adjusted by loosening the bolts which are accessible through holes in the metal shield between the prop shaft & the floor sheet metal. After loosening the center support bolts you can use a pry bar between the prop shaft & floor to adjust the center support. If the alignment isn't correct the prop shaft will usually "snap over" into proper location with very little input.

If the condition is both noise & vibration felt through the floor, re-indexing the prop shaft to the differential yoke may have to be performed:

Mark the relative alignment of the propshaft, rubber coupling, and differential in the original position.
Number the nut and bolt pairs and their hole in the rubber coupling which join the rubber coupling to the differential (silver) and the nut and bolts which join the propshaft to the rubber coupling (black).
Remove rubber coupling from differential and propshaft, ensure the nut and bolt pairs are kept together.
Re-assemble indexing the rubber coupling and silver bolts clockwise (relative to differential), and the propshaft and black bolts counter clockwise (relative to differential).
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
 

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Discussion Starter · #5 · (Edited)
Im pretty sure that I couldn't capture the sound with a camcorder... its low in tone and there is a feel with it as well. It is not a boom that makes you think its a violent transition.. a low rumble sounds about right. Maybe a low boom, with a slight shudder for a split second. Nothing like a bang or anything like that. Is this normal? I guess the thing that throws me is that it is only consistently noticable there... the other times it engages it is not nearly as noticable.

Thoughts? Thanks!.
EDIT: The sound is not constant, only happens for a split second... maybe more of a quick shudder than than a rumble.
 

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Im pretty sure that I couldn't capture the sound with a camcorder... its low in tone and there is a feel with it as well. It is not a boom that makes you think its a violent transition.. a low rumble sounds about right. Maybe a low boom, with a slight shudder for a split second. Nothing like a bang or anything like that. Is this normal? I guess the thing that throws me is that it is only consistently noticable there... the other times it engages it is not nearly as noticable.

Thoughts? Thanks!.
EDIT: The sound is not constant, only happens for a split second... maybe more of a quick shudder than than a rumble.
Mine does the same thing.
 

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it sounds like its normal. Mine has a slight rumble/boom when i hit the gas a bit harder to accelerate. lasts for about 3/4 sec.
 

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Same boom (I think) here, only when I'm moderate to harder on the throttle then suddenly back off--then it does it. It feels like a side effect of the sudden throttle shut and AFM, per the tune, wanting to kick in with that throttle let off...or at least I've thought so.

I have no vibrations or other sounds when cruising, etc., just that "boom" as said from the front drivetrain area under these throttle conditions--moderate/hard-->off or suddenly very light, then just for a second. I wondered if it was something that didn't start until after my VMS tune, and I think that might be so, but this far after can't remember vs. stock.
 

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Discussion Starter · #10 ·
Sounds like this is common, so I won't sweat it. I'll reference the bulletin posted, but I dont think that's it...

Is there any long-term effects as far as reliability in shutting off the AFM? Do those components need to be activated from time to time?

Thanks guys for all the quick responses here. The dealer I bought the car from is a smaller one and when I called to make the appointment with their service center, the guy acknowledged they hadn't sold many G8 GTs and had only "worked" on a couple... not exactly the picture of confidence. The input is appreciated!
 

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Like whitegreat8, I turned the AFM off with the Cortex tune. I couldn't stand the vibration anymore. Some cars are worse than others, and mine was bad. Not missing t at all.
 

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Sounds like this is common, so I won't sweat it. I'll reference the bulletin posted, but I dont think that's it...

I called to make the appointment with their service center, the guy acknowledged they hadn't sold many G8 GTs and had only "worked" on a couple... not exactly the picture of confidence. The input is appreciated!
Yep I have an 09 and I'm experiencing the EXACT same thing. Did they find anything "wrong" that could fix this?

Kinda sounds like it might be coming form the rear of the car to me. I thought it was something going on with the rear differential the first time I heard it. However,, it is definitely related to or instigated by the AFM system.

Thanks, KG
 

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Discussion Starter · #13 ·
Yep I have an 09 and I'm experiencing the EXACT same thing. Did they find anything "wrong" that could fix this?

Kinda sounds like it might be coming form the rear of the car to me. I thought it was something going on with the rear differential the first time I heard it. However,, it is definitely related to or instigated by the AFM system.

Thanks, KG

Dealer said it was normal... I turned off the AFM option with my superchips cortex and haven't looked back. I get about 1/2 to 1 mpg less on average... no big deal to me.
 

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Dealer said it was normal... I turned off the AFM option with my superchips cortex and haven't looked back. I get about 1/2 to 1 mpg less on average... no big deal to me.
Thanks for the reply. I figured they would say it was "normal". I've been bumping into manual when I'm in a 45mph zone. Doesn't seem to happen on the interstate. Thanks again,, I guess I'll be killing the AFM option when I get HP-Tuner for daily driving,,, and maybe cut it back on for any long road trips. KG
 
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